Method of producing carbon fibers

ABSTRACT

A METHOD IS PROVDED FOR PRODUCING CARBON FIBERS HAVING A TENSILE STRENGTH OF MORE THAN 150 KG./MM.2 AND A YOUNG&#39;&#39;S MODULUS OF MORE THAN 32,000 KG./MM.2 BY APPLYING A VIBRATIONAL ENERGY TO ORGANIC FIBERS UNDER A TENSION IN AT LEAST ONE OF A DRAWING STEP FOR OBTAINING THE PRECURSOR SUITABLE FOR THE PRODUCTION OF CARBON FIBERS, A PRETREATMENT STEP FOR FACILITATING CARBONIZATION OF THE PRECURSOR, A CARBONIZATION STEP FOR SUBSTANTIALLY CARBONIZING THE PRECURSOR AND A GRAPHITIZATION STEP FOR GRAPHITIZING THE CARBONIZED FOBERS AND FOR IMPROVING YOUNG&#39;&#39;S MODULUS.

3 705 236 METHOD or PRODUClNG CARBON FIBERS Toshikatsu Ishikawa and-Haruo Teranishi, Tokyo, Japan, gssignors to Nippon Carbon CompanyLimited, Tokyo,

apan No Drawing. Filed Oct. 28', 1970, Ser. No. 84,890 Claims priority,applifiition Japan, Nov. 1, 1969,

Int. Cl. coin 31/07 US. Cl. 423-447 3 Claims ABSTRACT OF THE DISCLOSUREThe present invention relates to a method of producing carbon fibersfrom organic fibers as a precursor. More particularly, it relates to amethod of producing carbon fibers having a high Youngs modulus.

The term organic fibers" used herein includes rayon fiber,polyacrylonitrile fiber, vinylon fiber, pitch fiber and other syntheticfibers as well as natural fibers. That is, fiber-like organicsubstances, which can be carbonized or graphitized by a heat-treatment,can be used as a precursor for carbon fibers. At present, theabove-mentioned fibers are practically used as the precursor.

Moreover, the term carbon fibers used herein means carbonaceous fibersand graphitic fibers.

A method for producing carbon fibers is generally as follows:

The organic fibers are previously heat-treated at a relatively lowtemperature of ISO-300 C. in an oxidizing atmosphere such as air,oxygen, ozone, chlorine and the like or in a non-oxidizing atmospheresuch as nitrogen and the like, or said fibers are impregnated or coatedwith a compound containing nitrogen or phosphorus or both and thenheated at the above described temperature as a pretreatment, whereby thefibers are gradually thermally decomposed. Then, the thus treated fibersare heattreated (carbonization) up to 1,000 C. in vacuo or in an inertgas atmosphere (non-oxidizing atmosphere) and further, if necessary,heat-treated (graphitization) up to a temperature of about 2,500 C. to3,000 C. whereat crystal structure of graphite is formed. Besides, theorganic fibers may be drawn in the presence of a heat medium such assteam and the like prior to said pretreatment.

The carbon fibers obtained by said method may be used as such andfurther they may be used as a reinforcing material for compositematerials, in which synthetic resin or metal is used as a matrix to forman improved composite material, which is light but has a high mechanicalstrength.

The carbon fibers are often compared with glass fibers, asbestos and thelike, but they are characterized by light weight, excellent heatresistance, good thermal conductivity, excellent lubricating propertyand high Youngs modulus. By using carbon fibers having a particularlyhigh Youngs modulus, improved composite materials having a highestspecific modulus can be obtained. Therefore, it is very important thatthe carbon fibers used for obtaining such composite materials have highstrength and United States Patent 0 3,705,236 Patented Dec. 5, 1972 r0CC Youngs modulus. In order to obtain carbon fibers having such a highYoungs modulus, an improvement of the above-described general method ofproducing carbon fibers has been proposed, wherein the organic fibersare heat-treated under a tension in the pretreatment step. In thisimproved method, however, when a load of 40 mg./d. is applied (the loadapplied on fibers of 2.5 deniers is 10 g.), the length of the fibersshrinks 12% and the obtained carbon fiber shows a Youngs modulus as lowas 27,000 kg./mm. while when a load of 160 mg./d. is applied (the loadapplied on 100 fibers of 2.5 deniers is 40 g.), the length elongates 36%and the obtained carbon fiber shows a Youngs modulus of 40,000 kg./mm.In such a process, a tow of 10,000 to 500,000 deniers is frequently usedas a precursor for carbon fibers, so that in order to obtain the load of160 mg./d., said tow must be subjected to a load of 1.6 kg. to 80 kg. Itis very difficult to uniformly apply such a load to each fiber of thetow in the heat-treatment step, particularly in a carbonization orgraphitization step. Furthermore, when such a large load is applied onlong fibers uniformly and continuously, the fibers are liable to bebroken and therefore such a process is not preferable.

An object of the present invention is to provide a method of producingcarbon fibers having a uniform and high Youngs modulus under a low loadwithout applying a high load on the above mentioned organic fibers.

Such an object is accomplished by applying a vibrational energy toorganic fibers in the axial direction or the transverse direction undera tension in a part or whole of a series of heat-treatment steps forconverting the organic fibers into carbon fibers. According to thepresent invention, the tension must be applied in order to sufficientlytransmit the vibrational energy to the fiber. This tension is preferredto be from 20 mg./d. to mg./d. When the tension applied to the fiber isless than 20 mg./d., the combination with the vibrational energy isinsufficient and a satisfactory eifect in each step cannot be expected,while even if the tension is increased to more than 120 mg./d., theeffect does not increase in proportion to the increment of the tension.The more preferable range of the tension is from 40 mg./d. to 100 mg./d.By applying the vibrational energy, carbon fibers having a more uniformand higher Youngs modulus than that of carbon fibers obtained only byapplying a high load, such as 160 mg./d. can be obtained under a lowload, for example, 40 mg./d. to 100 mg./d. without applying the abovedescribed high load. As mentioned above carbon fibers having a highYoungs modulus are obtained only by applying a load as high as 160mg./d., but the merit of the present invention lies in that carbonfibers having a higher and more uniform Youngs modulus can be obtainedby applying the vibrational energy.

Particularly, according to the present invention carbon fibers having ahigh Youngs modulus more than 60,000 kg./mm. can be easily obtainedunder a relatively low load (e.g. 40 mg./d.) by subjecting the fibers toa heat treatment while applying vibrational energy. For instance,according to the present invention it has been found that Youngs modulusof 2 to 3 times higher than that of wellknown carbon fibers obtainedunder a high load can be attained.

In the present invention, the vibrational energy is applied in aheat-treatment step heating at a relatively low temperature of C. to 300C. (a pretreatment step), a heat-treatment step heating up to about1,000 C. (carbonization step) and a heat-treatment step heating at atemperature of 2,500 C. to 3,000 C. (graphitization step). Furthermore,the vibrational energy is applied in a step wherein organic fibersobtained by a wet or dry spinning process are drawn in boiling water orsteam of 100 3 C. to 140 C. or a solvent such as polyethylene glycol,

glycerine and the like heated at 100 C. to 180 C. to improve degree oforientation, whereby organic fibers suitable for production of carbonfibers are obtained. The steps for producing carbon fibers according tothe present invention can be divided into the following four steps:

(1) drawing step (step for obtaining the precursor suitable for theproduction of carbon fibers),

(2) pretreatment step (step for facilitating carbonization ofpolyacrylonitrile fiber, pitch fiber and the like),

(3) carbonization step (step for carbonizing substantially theprecursor), and

(4) graphitization step (final step for graphitizing the carbonizedfibers and for improving Youngs modulus).

According to the present invention, the vibrational energy may beapplied over the whole steps or in any one of the above four steps.Furthermore, the vibrational energy may be applied over any two or threesteps of the above described steps. The vibration is desirable to beapplied over the whole steps, but the time for applying the vibrationalenergy can be optionally selected depending upon kind and properties oforganic fibers to be used.

The effects obtained by applying the vibrational energy in each step forproducing carbon fibers are as follows:

(1) By applying the vibrational energy in the drawing step, the fibershaving a high elongation percentage, an improved degree of orientationand a fine diameter can be obtained, which are preferable for theproduction of carbon fibers.

(2) In the pretreatment step, organic polymeric fibers such aspolyacrylonitrile fiber, pitch fiber and the like are softened at atemperature of about 200 C. to 300 C., so that if the vibrational energyis applied in such a stage, the drawing ratio increases, the diameter ofthe fiber becomes fine and at the same time the inner deficiency of thefiber such as void, flaw and the like is removed, whereby the drawingeffect appears noticeably.

(3) The application of the vibrational energy in the carbonization stepis also effective. Generally, the carbon fiber in the carbonization stephas a cross-linked structure, so that the drawing is very diificult.However, when the vibrational energy is applied in accordance with thepresent invention, it is possible to draw the fiber and the strength andYoungs modulus can be improved.

(4) The carbon fiber in the graphitization step is deformed as in aplastic at an elevated temperature above 2,500 C. and when thevibrational energy is applied, the drawing is carried out uniformly andsmoothly and consequently the application of the vibrational energy hasa remarkable effect for improving strength and Youngs modulus in theobtained product.

The frequency of the vibrational energy to be used in the presentinvention is from cycle/sec, preferably 30 cycle/sec. to 1,000,000cycle/sec. In the case of less than 10 cycle/sec. or more than 1,000,000cycle/sec, an appreciable effect for improving Youngs modulus anduniformity is not obtained.

Means for applying the vibration includes a mechanical means, anelectrical or electromagnetic means, a sonic means and the like.

Furthermore, there are the following methods for transmittingvibrational energy:

(1) A load is suspended from one end of the fiber and a vibrator isconnected to another end, and the fiber is subjected to an axialvibration during the heating step.

(2) A load is suspended from one end of the fiber and another end of thefiber is fixed and a vibrator is contacted with the intermediate portionof the fiber to be heat-treated, whereby the fiber is subjected to avibration.

(3) In a continuous process wherein the fiber is extended betweenrollers and is heat-treated at the intermediate portion thereof, one orboth of said rollers are subjected to a vibration, which is transmittedto the fiber through the roller.

These methods may be optionally adopted insofar as the aimed purposesare the same.

According to the present invention, the duration for applying thevibrational energy is preferably from 5 minutes to 10 hours in each ofthe above steps for producing carbon fibers. Even in the case of lessthan 5 minutes, some effect can be attained, but the improvement ofuniformity and properties of the fibers to be treated is not sufficient.Furthermore, in the case of more than 10 hours there is no advantage inview of an effect. Rather, when the vibration is applied for too long aperiod, the yarn breakage of the fiber to be treated occurs frequently,so that such an application is not preferable.

In general, the carbon fiber shows a higher strength as the diameterthereof decreases. Presumably, this is based on the fact that thediameter of the fiber is reduced by drawing and the like andconsequently, the deficiency such as voids and the like decreases and atthe same time fibrous molecules approach with each other and the degreeof orientation is improved. The method of applying the vibrationalenergy to the fiber according to the present invention has a remarkableeffect for considerably reducing the diameter of the fiber and such amethod will contribute to the approach of fibrous molecules and theimprovement of degree of orientation and consequently, carbon fibershaving higher strength and Youngs modulus are obtained.

The following examples are given in illustration of this invention andare not intended as limitations thereof.

EXAMPLE 1 A bundle of 100 polyacrylonitrile fibers, each of which havinga fineness of 2 deniers, was suspended in a vertical electric furnaceand a load of 16 g. mg. per denier) was applied to the lower end of thebundle and then a vibration of 50 cycles/sec. was applied to the bundleby means of an electromagnetic vibrator in air While maintaining thetemperature at 200 C. The variation of the fibers are shown in thefollowing Tables 1 and 2.

TABLE 1.ELONGATION OF FIBER DUE TO VIB RATIONAL ENERGY Elongation(percent) Polyaerylonitrlle fiber, Vibration of treating time, hr. No.vibration 50 cycle/sec.

TABLE 2.PROPERTIES OF FIBER TREATED AT 200 0. FOR 8 HOURS Vibration Noof 50 vibration cycle/sec.

Diameter of fiber after treated, a 17. 2 12. 5 Reducing ratio based onoriginal length of fiber,

percent 14. 4 37. 8

Tensile strength, kg./rnm. 13 22 Youngs modulus, kg./mrn. 630 880 Then,the fiber was graphitized in an inert gas atmosphere to obtain a resultas shown in the following Table 3, in which the fiber treated withvibration was excellent in Youngs modulus and particularly in tensilestrength.

EXAMPLE 2 Polyacrylonitrile fibers with a purity of 94% (fineness: 2deniers, a bundle: 3,000 fibers), which had been obtained by extrudingthrough nozzles and coagulating into filaments, drying and drawing 6times their original length in hot water, were drawn in a steam of 110C. under a tension of 100 mg./d. While applying a vibration of 10,000cycles/sec. according to the present invention. In this case, themaximum drawing ratio was 3 times, by which the fibers were drawn 18times their original length. For the comparison, when the drawing wascarried out without using vibration under the same condition asdescribed above, the maximum drawing ratio was only 2 times and thedrawing ratio based on the original fibers was 12 times and in case ofmore than 2 times the drawing was impossible because yarn breakageoccurred.

These fibers were heat-treated up to 250 C. under a tension of 40 mg./d.and then up to 1,000 C. at a rate of 50 C./hr. and finally up to 2,800C. in about 2 hours to obtain carbon fibers having properties as shownin the following Table 4.

TABLE 4.PROPERTIES OF FIBER TREATED AT 2,800 C Present Control invention(no vibration) Tensile strength (kg/mm!) 200 120 Youngs modulus (kg/mini42, 000 26, 000

EXAMPLE 3 Polyacrylonitrile fibers of 1,000 deniers were heattreated inan electric furnace at 230 C. under a load of 40 mg./d. while applying avibrational energy of 50 cycles/sec. The properties of fibers aftercarbonization at 1,200 C. and fibers after graphitization at 2,700 C.,when applying the vibrational energy for 0.5, 1 or 2 hours, are shown inthe following Table 5.

From the above table, it can be seen that in any case the application ofthe vibrational energy is considerably effective for reducing thediameter of fibers and improving Youngs modulus.

In this example, the heat-treatment was continued without applying thevibration after the vibration was applied for the given time asdescribed in Table 5. When the vibration was applied, length of thefiber elongated but when the vibration was not applied, the fibersshrunk and after heated at 230 C., about of elongation was observed.Even the resulting carbon fibers applied the vibration only in theheat-treatment at 230 C. showed a Youngs modulus of 60,000 kg./mm.

It can be seen from this example that the effect of vibration isobtained even if the vibrational energy is not applied over the wholesteps for thermally decomposing the organic fiber.

' EXAMPLE 4 A bundle of 900 viscose rayon filaments having 1,400deniers, said viscose having a polymerization degree of 450, washeat-treated from 150 C. to 300 C. at a rate of 10 C./hr. in nitrogenatmosphere and then heattreated up to 1,000 C. at a rate of C./hr. undera tension of 50 mg./d. while applying a vibration of 15,000 cycle/sec.and further heat-treated at 2,800" C. The properties of the thusobtained carbon fibers are shown in the following Table 6.

TABLE 6 Tensile Young's Elongation strength modulus (percent) (kgJmmJ)(kg/mm!) Present invention 6 150 82, 000 Control (under the samecondition without applying vibration) 0. 5 80 24, 000

EXAMPLE 5 TABLE 7 Tensile Youngs Elongation strength modulus (percent)(kg/mm?) Present invention 28 186 45, 000 Control (under the samecondition without applying vibration) 10 22, 000

What is claimed is:

1. In a method of producing carbon fibers having a Youngs modulus ofmore than 32,000 l g./mm. and a tensile strength of more than 150lag/mm. from organic fibers as a precursor comprising (a) drawing theprecursor suitable for the production of carbon fibers,

(b) subjecting the drawn precursor to a heat treatment in an oxidizingatmosphere,

(c) carbonizing the oxidized precursor in a non-oxidizing atmosphere andfurther,

(d) graphitizing the carbonized fibers, the improvement consisting ofapplying a vibrational energy of from 10 cycles/ sec. to 1,000,000cycles/sec. to said precursor or fiber under a tension of from 20 mg./d.to mg./d. in at least one of said steps (a), and

2. The method as claimed in claim 1, wherein the time for applying thevibrational energy is 5 minutes to 10 hours.

3. The method of claim 1 wherein said vibrational energy is at least 30cycles/sec.

References Cited UNITED STATES PATENTS 3,533,743 10/ 1970 Prescott eta1. 23-209.1 2,745,136 5/1956 Deboutteville 264167 X 3,538,206 11/ 1970Hann 264-70 3,552,923 1/ 1971 Carpenter et al. 23--209.1

EDWARD J. MEROS, Primary Examiner US. Cl. X.R. 264--29, 69

